The present invention relates in general to phase lock loops and, more particularly, to a bias generator for a voltage controlled oscillator in a phase lock loop.
A conventional phase lock loop (PLL) generally includes a phase detector for monitoring a phase difference between an input signal and an output signal of a voltage controlled oscillator (VCO). The phase detector generates an UP control signal and a DOWN control signal for a charge pump to charge and discharge a loop filter at a loop node at the input of the V CO. The loop voltage developed across the loop filter determines the output frequency of the VCO. The UP and DOWN control Signals driving the charge pump set the proper loop filter voltage at the input of the VCO to maintain a predetermined phase relationship between the signals applied to the phase detector.
PLLs are widely used in data communications, local area networks in computer applications, microprocessors and data storage applications to control data transfers. PLLs are often implemented with Complementary Metal Oxide Semiconductor (CMOS) technology to provide low cost and low power operation allowing a system designer to extend the tracking range and frequency-aided acquisition.
A common problem related to VCO operation is ground bounce which causes undesirable UP and DOWN pulses from the phase detector and results in voltage fluctuations at the loop node. Ground bounce is typically caused by other activity on the integrated circuit such as digital bus switching. The ground bounce can induce a short duration swing, say 5 ns, in the power supply potential to the phase detector, charge pump and loop filter capacitor, which generates UP and DOWN pulses unrelated to the phase error between the input signals to the phase detector. The UP and DOWN pulses related to ground bounce cause movement in the loop node and undesired changes in VCO output frequency. The PLL must wait for the phase detector to correct for the improper VCO output frequency in its normal fashion. Unfortunately, ground bounce often results in erroneous VCO operation.
Hence, a need exists to maintain the VCO output at a frequency determined solely by the phase error to the phase detector independent of external influences like ground bounce.